1. Field of the Invention
The present invention relates to a surface-mountable PTC (positive temperature coefficient) thermistor, and a method of surface-mounting such a PTC element onto a mounting target such as a substrate.
2. Description of the Related Art
In FIGS. 13 and 14, a perspective view and a front view of a conventional surface-mounting PTC thermistor are illustrated, respectively. The surface-mountable PTC thermistor includes top and bottom surfaces of an element body 1 that have a round button shape, electrodes 2 and 3 and terminals 4 and 5, which are connected to each of the electrodes 2 and 3 respectively, and are bent extending downward in a hooked configuration. The PTC elements are shipped and sold in a package consisting of an embossed tape or other similar packaging. As shown in FIG. 15(a), the PTC elements are picked one by one by a sucking nozzle 6, which defines an element holder that is part of a mounting apparatus, and the PTC elements are mounted on a mounting target so as to be subsequently fixed by soldering. The PTC element is mounted such that a space s is formed between the element body 1 and the substrate 7 in order to prevent heat transfer from the element body 1 to the substrate 7.
In the above-mentioned conventional structure, when mounting a PTC element onto a predetermined position of a substrate, the PTC element is pressed onto the surface of the substrate 7 by an appropriate force together with the sucking nozzle 6 itself so that each of the terminals 4 and 5 is firmly contacted with the solder (not shown in the figure) disposed at the mound of the circuit pattern.
In this case, each of the terminals 4 and 5 is connected near the outer edge of the upper and lower electrodes 2 and 3 of the element body 1, and the vertical leg portions 4b and 5b of each of the terminals 4 and 5, respectively, is placed outside the element body 1. This results in concentration of stress acting in the direction of detachment on the junction portion 4a of the upper terminal 4 with the electrode 2 caused by the reaction force acting against the pressing load on the substrate. Thus, the junction of the upper terminal 4 might be broken as shown in FIG. 15(b). Therefore, the pressing is to be adjusted to the junction strength of the electrode 2 and the junction portion 4a, thereby making it difficult for adjustment, because even elements having the same specifications have a different junction strength.